Over the past several years, a significant increase in recording density in thin-film magnetic recording discs has been achieved, and there is a continuing effort to increase recording density further.
A number of magnetic properties in a thin-film disc are important to achieving high recording density, including high coercivity and remanance, and low flying height. The latter property is important because, as the read/write head is moved closer to the disk, there is less overlap of voltage signals in adjacent magnetic domains in the disc, with a corresponding increase in recording density.
Heretofore, disks having high coercivity and remanance characteristics have been prepared by sputtering a thin magnetic film on a metal substrate, typically an aluminum substrate. Prior to sputtering, the substrate is plated with an alloy plating, such as a nickel/phosphorus plating to achieve a requisite surface hardness. The plated disc is then polished to remove surface nodules which form during the plating process.
The plated substrate may be textured by abrading or sanding, using a rotary abrasive pad placed off center with respect to the surface of the spinning substrate, or by etching with acid. The purpose of the texturing is to create a roughened surface characterized by submicron surface irregularities. The roughened surface reduces stiction (static friction) between the disk and head by reducing surface contact between the two, particularly for start/stop cycles.
The sputtering operation used to produce the thin magnetic film is preferably carried out by first sputtering a chromium underlayer onto the substrate surface, then sputtering a cobalt-based magnetic thin film over the underlayer. A protective, lubricating carbon overcoat is applied over the thin-film layer by sputtering.
Despite the favorable magnetic and surface-wear properties which can be achieved in the above-described thin-film magnetic disc formed on a metal substrate, the recording density of the disc is limited in flying height by the irregularities on the disk surface (due to the surface texturing and surface irregularities related to the plating operation). In addition, surface peaks can appreciably increase the wear on the disc, and on the read-write head.